Drilling mud resistivity meter



`lune 3, 1952 P. P. REICHERTZ 2,599,413

DRILLING MUD RESISTIVITY METER Filed Jan. 18, 1950 3 Sheets-Sheet l Q (Y) we x9 Q l /f In' 'Hl'. wl in". l I EHRQ, (J w N 'HHH 5 Q M .'n l l l a n; q) ln q) K LL* o5 N 7 1 N \^H I l l\ N\ QN un? I K I N QN u is N n m "sa N mi N www @kx A 5 LL, Pauw P. Rezclzem;

' INVENToR.

BY du, @MMM AGENT June 3, 1952 P. P. REICHr-:RTZ

DRILLING MUD RESISTIVITY METER Filed Jan. 18, 1950 5 Sheefcs-Sheet 2 IN VEN TOR.

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AGENT Junes, 1952 A P'RRE'CHERTZ 2,599,413

DRILLING MUD RESISTIVI'IY METER Filed Jan. 18, 1950 5 Sheets-Sheet 5 Fie. 4

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BY A@ @Aw AGENT Patented June 3, 1952 rApp'xicationlliammry 18, 19504'seria1`No. 139;268

"This invention "relates"fgeneral1y to "the measurement #ofchamerensnes fuids'fand more particu'leii'lyv to ameter Y'for measuring f directly "the speciiic y"electric'zal `"resistivity "a 'ii'uid such "as liquids 'collected' from' Wellsv 'or 'mulls used "in Iconnectiomw'ithnrilling.

There has "been l'found to be "a 'considerable need for 'infomation ons theele'ct'rical properties of'fdril'lii'igmuds,'-drillingiiuid ltrates','"formation waters.' jandcor'e'sarnples in interpreting data' obtained from electrical Well logging techniques.

i -Electrical 'resistivity rand 'selfepotentiallogs are usedftoexaminerocks"and formations penetrated bra" bore i hole andare important' as an aid" in theiocationr v*of sectionsrin 'a particular bore or well `VWhere fthe :production of 'oil Amight be' expected-and, just asimportant in well'c'ompletion practices, the'iocation' VofI sections Whereethe lproduction 'idees`r not appearpossible. lIn4 obtaining resistivity" curves, an Aelectrode or Velectrodes' are lowered into af borehole, current ispassedtherefrom'I tothe-j formations, and Ea'potr-:lntial drop; Ldue to :iio'w ofT that current is measured. j The mud or liquid i in f the `'hole unavoidably forms a *part 'of the 'currentjpatir'and Vthus hasta direct 'effect upon differ yfcon's iaeralily from v'the v*true resistivity of the formations.'V 'By making such 'af'correction' in apparent resistivity a maior variable independent-oi 'formation isfremov'ed. Y

' fIn rselfpoten'tial "nieasurements, it #has 'been foundv empirically f'thatjin the v(ifuli Coast "area, sand formations' generally *arenliaracterizediby 'a negative I*self'''potential 4as` comparedtoaidi acent s'hales. -HoiiveverI Where 'the formation Water i'nth'e sand is Nless =saline 'arid consequently "of lower rresistamae' than the mud-in tliehorefth'e selfpotential-"difl'erence"may bepositive.

*"Ihus; there is cemparent"the nee'd for mud'and formation -fluidresistivity' data' for interpretation of1 resistivity `and "self-potential' logs. y

vIn"ac :ordaiicewith the -presentinveiitiom 'there isL provided an apparatus" for measuring "the vresistivityof' liquids endfcomprisesa circuit 'includin'gf'faiiqud sample'cellra" nxedijresistancefa"pair f-`eiectrodes "immersed inthe' `v`celltand"nieansior controlling? the "current ilow-tl'iroughithe circuit. Anuli detecting 'circuitis provided foricomna'ring the notent'ial drop `-'across the iixed resistance produced bythe' flowA f-a known-'current tl'iei'enimm'gir-"simil1 the potential-eropfbetweenthereiec trdes `inf-thc fceii :andi inciudes-v--wscnrceoifnoten'til-*in'ohase `4'opposition to tnei'potentialdp "roiaims. -(Cl. 17a-183) lf2 Y across'-thelxdresistarice andbetweentheelec trodes'and a' potentiometer: for varying the magnitude A(")f'lhe' potntialfrom the potential source tobalance the.nllirneasuringfcircuit -In apreferred form of the inventiomfa'potentiometerTacouratelycalibratedy in terms ofspeciiic resistivityfof tnenuid in the een is-utiiized-to=vary the fpotentialfromthe potential-sourcet `For""further objects and advantages of the-"invntion "and -for "a, more complete description thereof, reference may' now beha-dto tlie`-fol1ow ing description 'taken lin "conjunction with vthe accompanying drawings 'in Ywhich:

'Figxl isa: schematic ldiagram oit-heinvention;

Fig. `3' is a detailed schematic diagram of "-tlie iFig: 4 isfafview of si. portable-'meter incorporating the-present invention. v

l"ZEtef'erring"nowito'Fig. 1,-there'has been illustratedfacellfl 0 containinga 1samplevr of liquid l H, the'resistivity of -Whichitis desired Ato f measure. The Asample-cell f is provided with current electrodes i2 and' 'f 3'and potential Y electrodes I 4=jand i 5. The' resistivity 'o'f the liquid inlthecell' I lll-is determined frby al potentiometr-iclmeasurement 1in -Whicha-knowxi-current is caused toow lthrough thel liquid 'between the current electrodes iI--Ziand [3ra/nd# the Apotential drop Aacross that portion'ffof the'fliduid `between the electrodes M 1and`I'51iis,-f-in efiectjmeasured.

'As "illustrated, f there iis provided xanioscillator 20'1Who`seo'utput terminalsl aree connected toi-the primaryiwindingiurof fa, transformer/122. Transformer322 yis provided WithftWosecon'daryWindings,l ione-fof"tirliic1f1,fthe'A secondary Windingll is connected-'to the cell f. More'particularlyfoonducten-'125 l is-connected'f 'f rom theA upperterminal of 'the winding '23 to the f current electrode SP2. 'Ilnelower terminal ff'winding 23 -is connected by Way Ao'fconductorf'z'lffxed resistanceZB; varialle resistance l"29 "and: conductor '530 f to the I-currerit electrode-T3. "Wherrthe oscillator-120 or'anfquivalentfsource 'of i alternating currents -is energized, current 'flows #throng-h' the sample between electriodesl |2 and T13. The magnitude ofthe-current is determined bythe'fvoltage"induced-in Wind-nig 23fthe!rsistamos'ean'd-Z91 andfthe'iesstance oi'eredbythelliquidlinlthe dell Hl.

tor 40 to a first input terminal of the null detector 36. Conductor 4I connects the lower terminal of the potentiometer to a double-pole, double-throw switch 45 and thence by way of conductor 46 to the second input terminal of the null detector 36 and to an electrical ground.

The null detector 38 and the potentiometer 31 having a reference voltage from secondary 35 applied to the terminals thereof is utilized to measure or determine, first, the magnitude of the current flowing through the cell I0, and second, the magnitude of the potential drop between` the electrodes I4 and I5. The above current and voltage measurement immediately suggest an Ohms law calculation of a resistivity function of the liquid in the cell I0. As will later be more completely explained, the calculation itself is accomplished by proper construction and operation of the above-described basic circuit.

The measurement of the current is accomplished by connecting the resistor 28 into the input circuit of thenull detector 36 with the potential drop thereacross in phase opposition to the potential across the variable portion of the potentiometer 31, i. e., between thetap 38 and conductor 4I. More particularly, conductors 50 and 5I connected to the terminals of resistor 28 are connected to one-pair of terminals of the doublepole, double-throw switch 45. When the switch is in the left-hand position, the input circuit of the null detector 36 includes, in series, the resistance 28 and the selected portion of the potentiometer 31.- Thereupon, the position of the tap 38 may be varied until a potential point is reached that is equal and opposite the potential drop across the resistor 28 thereby to produce a voltage null in the input circuit of the amplifier 36 which will be indicated on meter 52. Alternatively, the tap 38 on potentiometer 31 may remain fixed and resistor 29 varied changing the cell current to produce a similar null indication.

After a first voltage null has been obtained, switch 45 may be shifted to the right-hand position in which the electrodes I4 and I5 and the liquid therebetween is in the input circuit of the amplifier 36 and in series with the portion of the potentiometer 31. More particularly, conduetors55 and 56 connect electrodes I4 and I5 to the right-hand terminals of the double-pole, double-throw switch 45. In effect, the resistance of the liquid between electrodes I4 and vI5 is substituted-for the resistance 28 as respects the input circuit of amplifier 36. Thereupon, the tap 38 of potentiometer 31 may be varied until a second null is obtained. Theratio of the values of the voltages tapped off from the potentiometer 31 for the production o f the two voltage nulls in the input circuit of the detector 36 is directly proportional to the resistivity ofthe fluid contained in the cell. Thus, regardless of the magnitude of the current in the cell upon productionof a first voltage null, the ratio obtained by making two null measurements and noting the voltages required to produce the nulls 4(or, A the ratio of the resistance between the tap 38 and conductor 4I), a resistivity function for the liquid in the cell may be obtained. y y

Fig. 2 illustrates a modification of thel portion of the circuit of Fig. 1 by -which the voltage from the oscillator is derived for use in the cell circuit and in the null indicating circuit. As illustrated in Fig. 2, the twin secondary transformer 22 of Fig. 1 has been replaced by a single secondary transformer 22. In this modification, the potentiometer 31 is connected across the primary of transformer 22 as by conductors 6U and 6I. In general, the present invention requires only that the voltages used in the cell circuit and in the null detecting circuit be of the same frequency and synchronized; that they are not conductively associated except through the one common link necessary for measurement (i. e., through resistor 28 or electrodes I4 and I5) and that they be mixed in the input circuit of the Aamplifier 36 in phase opposition. If the foregoing requirements are satisfied, any desired source and circuit arrangement for production of same may then be used.

Referring now to Fig. 3, there is illustrated in greater detail the elements and circuits diagrammatically illustrated in Fig. 1. The cell I0 may comprise an elongated tube of insulating or nonconducting material (illustrated in section) having embedded in the walls thereof the electrodes I2-I5 in the form of metallic rings suitably spaced one from the other. The tube I0 may then be provided with a stopper or plug for the bottom to retain liquids therein during measurement. The inner surface of the electrode rings conform with the inner surface of the cell cylinder to permit it readily to be cleaned, thus facilitating theY operation of the instrument.

The oscillator 20 is illustrated as being of the resistance-capacitance (RC) type and includes a pentode 10 having in the grid circuit thereof a resistance-capacitance network including resistances 1I14 and condensers 15-18. Energy from the anode of tube ,10 is fed into the grid RC network by way of conductor 18 to provide the necessary feedback to sustain oscillation. Oscillators of this general type and equivalents thereof are Well known in the art and may be readily constructed by reference to suitable handbooks. The frequency of operation is preferably in the low audio range, for example 100 C. P. S., though not limited thereto. In the particular oscillator illustrated in Fig. 3, a rectifier 88 and the resistance 8I and condenser 82 stabilize the output of the oscillator,l acting to regulate or control the effective grid bias provided by the bias battery 83 connected in the cathode circuit of the tube 10. The potential rectified through the diode in the grid circuit of the oscillator tube 10 is, of course, proportional to the magnitude of the output energy fed into the grid circuit by way of conductor 19 and is of polarity such asto increase the negative bias. i. e., additivewith respect tothe battery 83, thereby to 'decrease the gain of the tube A1l! when the output thereof tends to 'increase and thusprovides a measure of amplitude stabilization forthe 'oscillator output.

The voltage appearing at the tap 38 of the potentiometer 31 is coupled by way of conductor 48 to the input grid of tube 85 of the amplifier 36. In the modification illustrated in Fig. 3, the amplifier 36 includes tubes 85, 86, and 81, whose cathodes are directly coupled in a common circuit and connected to the positive terminal of battery 83 to be normally biased for amplification of input signals. Tubes 85, 86 and 81 are of the pentode type, each having its lsuppressor grid coupled to its-cathode and its anode connected by wayof load resistor 84 to a source of B-supply (B+). 'Ihescreen grids of tubesv85 and 86 are connected .to B+ by Way-of screen resistors 84a. The screen grid of tube 81 is connected directly to B+. The output of thefinal tube r81'is fed to a rectifier 88 eifectivelyconnected between the anode 4,92 of tube 81 and ground.- In parallel with the diode 88, there is provided a feedlciack-indi-l eating-:circuit comprisewof resistor safmeterz andra#sI'noothingV 'circuit or filterincluding v'condensers190and resistor-'9L 'Y'I'heoutput terminal of resistor ji!!! iis connected to ground fthrough resistor 9'2 'which forms-a part ofthefgriddinpedance for'itubefBB. For fai-"givenlevelf-'of fsignal applied' to 'the -in'putof tube'18'5, `'the output of' tube 81 is "rectified :fa-nd a constantdeiiection appears on meter 52 due to the smoothing actionoithe lter `comprising i the 'condcnsers 90 i and 'resistor 9 I..-: The tub'e'186 .preferably isfof the lr'e'mote cute oif itypeso Ithat i flow of 'the rectiefd "or direct current through the feedbackire'sistor'lSZ 'produces a-biasin'givoltage thereacro'ss'=additive linvpolari'ty, inthegrdbathode' circut of ftube *35,fwith th'e bias battery '83 -to- 'shiftthe "'operatin'g point #on the Eg-f-Ip f characteristic "of tube "586 Ito 'fa i point approaching fcutsoif. x

'Thus, for high values of Ainp'uts'fign'al,alla'rge voltage isy developed across res'istor 2921and'the meterf52 is thus protected rromffe'xcessivef oeflections. IA'simpractical-matter, the farnplicationf of the `vamplifier as a function of input fvoltage'is logarithmic, being veryfs'mallfffor :a l'arg'e 'input signal 'rand a maXi-mum-Whe'n' a `:voltage -null' is produced-'inthe input circuit. Thismansiihat potentiometer-'31 or resistor i29lma3f -befadjusted for a voltage'null very'ac'cura'telyptlie eect'of such variation at the criticalnull point l'being indicated 'with great'sensitivity; Thus, 'the Jac cura'cyof` the instrument' is dependent only upon the'a'ccuracy' of calibration of "1ooteritionieter 31'.

vkInfad'dition'to lthe DJC. feedback-above described, afsec'ond o'r alterna'ting'ourrentfeedback circuit lis 'utilized 'between the A'anode-92 lo'f tube 81fand tsgrid '93. Energy from'theandems fed` through fa; parallel-'T 1 lter network ISM which iszcoupled to the' anode' 95161 tube 86 andthnce byway ofl coupling 'condenseri Land 'grid-resistor 9'I-"i1n theinput'of tubel'l. J'Ih'epalrallel-"T'lter networkis-designed to attenuatein thev feedback loopV signals' of the 'same frequencyias those VVirorn the source 'oscillator' 20 and tos'u'stain inf-the feedback loop "all 'other frequencies. Thus, the' ampli-- e'rfhas "a 'loweiiective' amplification, d'ue-'to l'tlie negative feedback, of'all 'frequencies except-the desiredJ-signal.

-T-he foregoing utilization of twoseparatefee'c'iback -networks has the effect vvof making the ainplifier insensitive to anysignalexcept frornthe source-*210 and permitsthea'niplier to loe-"particularl'yf sensitive vto 'that signal 'only when a volt-Y ageenulll isxproduce'd in thelamplir -input'circu'it thereof.

The f'circits illustrated hei-ein and abo've de'- scri-bed` have been found to bei particularly desirabie'inf measurement of the resistivity `of valiqiii'd wherein'fa-rst voltage null is produced in the input'on the amplier 36 with the potentiometer then set iat-a `value -Whi'ch represents the 'current flowing through the rcell Il] an'dfor the detection ofi-a second voltage null, -Whenthepoi'tion of the fluid inthe cell between'the electrodes I4 andl replaces the resistor 128,by variation of the-'po sucheas'sodiumchloride,p'otassiumwenlorie,isulance 28 in the input circuit of amplifier 3E.

The tap 38 on potentiometer 31 is then varied until a second null is noted on meter 52. The numerical value of the potentiometer reading corresponding to the associated second null indication is then noted. This second null reading will then be the meter base or the starting point for all subsequent tests using the particular cell in the measurement of liquids of unknown resistivity to be placed in the cell I0. A

For example, once the above calibration has been performed and a meter base obtained, the measurement of the specific resistivity of an unknown liquid is accomplished as follows. The potentiometer 38 is set at the meter base as above determined upon calibration. The unknown liquid is placed in the cell I0, switch 45 is moved to the lower positionplacing resistance 28 in the input circuit of amplifier 36 and resistance 29 is then varied until a null is observed on meter 52. The cell current fiowing at such a null position is the same magnitude as the current flowing through the standard solution in the calibration of the instrument. Thereafter, vthe switch 45 is moved to the upper position and the potentiometer varied until a second null is noted. The null reading of the potentiometer setting cerresponds numerically with the specific resistivity of the unknown mud sample placed in the cell.

Though the invention has been illustrated by the above modifications thereof, it is to be understood that further modifications within the scope of the appended claims may now suggest themselves to those skilled in the art.

What is claimed is:

1. A system for measuring the resistivity of a liquid comprising a circuit including a cell containing said liquid, a fixed resistance, a source of alternating current and a variable resistance for controlling the magnitude of the current in said circuit, means for determining the adjustment of said variable resistance at which a predetermined current fiows through said cell comprising a null detector having in its input circuit said fixed resistance and a potentiometer in circuit with said source, only a predetermined portion thereof being effective in said input circuit so that a voltage of predetermined magnitude is in phase opposition to the potential across said fixed resistance whereupon flow of said current of predetermined magnitude will produce a voltage null -in said input circuit, means for replacing said fixed resistance in said input circuit by a portion of the liquid current path in said cell whereby variation of said potentiometer changing said predetermined portion produces a second voltage null in said input circuit, the ratio of said predetermined portions of said potentiometer corresponding with said voltage nulls being proportional to the resistivity of the liquid in said cell.

2. A system for measuring the resistivity of a liquid comprising a circuit including a cell containing said liquid, a fixed resistance, a source of alternating current and a variable resistance for controlling the magnitude of the currentin said circuit and through said cell, means for determining the adjustment of said variableV resistance at which a predetermined current fiows through said cell comprising a null detector having in its input circuit said fixed resistance and a source of potential of predetermined magnitude and in phase opposition to the potential across said fixed resistance whereupon fiow of said current of predetermined magnitude will produce a null in said input circuit, means for replacing said fixed resistance in said input circuit by a portion lof the current path in said cell and means for varying said potential from said predetermined value to produce a second voltage null in said input circuit whereby the ratio of said potentials corresponding with said voltage nulls is proportional to the resistivity of the liquid in said cell.

3. A system for measuring the resistivity of a liquid adjacent four spaced apart electrodes which comprises a current circuit including a source of alternating current, a resistance and two of said electrodes connected in series for flow of current through the liquid between said electrodes, a null detecting circuit including in the input circuit thereof said resistance and a source of potential in phase opposition to the voltage across said resistance, means for varying the magnitude of said potential to produce a null in said input circuit as a measure of said current. means for replacing said resistance in said input circuit with the second pair of electrodes and the fiuid therebetween to produce a second null upon variation of said potential as a measure of the voltage drop across said liquid, whereby the ratio of said potentials at said two nulls is proportional to the resistivity of said liquid.

4. A liquid resistivity meter comprising a four electrode sample cell, a resistance and a source of alternating current connected in series with the extreme electrodes of said cell for flow of current through the fluid therein, a potentiometer and a null detector connected in` series circuit with said resistance, a source of reference voltages connected across said potentiometer in outof-phase relation to the voltage across said resistance due to flow of said current for producing a first voltage null in said series circuit by variation of said potentiometer, and switching means for connecting said potentiometer and said null detector in circuit with the intermediate electrodes of said cell and the fluid therebetween for producing a second voltage null upon further variation of said potentiometer whereby the ratio of the value of said potentiometer settings at said null is proportional tothe resistivity of said fluid.

5. In a system in which an alternating current from a single frequency source traverses a circuit including in series a. resistance and liquid between a pair of current electrodes, means for measuring the resistivity of said liquid which comprises an amplifier having a potentiometer in its input circuit and an output circuit, a source of alternating current potential of said frequency connected across said potentiometer, a second pair of electrodes positioned in the path of current fiow in said liquid, switching means for selectively including in said input circuit said resistor or said liquid between said second pair of electrodes for nulling the voltage drops thereacross by variation of said potentiometer to reduce to zero the signal input to said amplifier, a negative feedback circuit connected to said output circuit responsive to the amplifier signal output for reducing the gain of said amplifier when said input signal is not zero, and filter means for selectively attenuating all frequencies in said amplifier except that of said single frequency source whereby said system has high sensitivity exclusively to the frequency of said source at said null points only and means connected to said output circuit for indicating the null adjustment of said potentiometer.

6. A system for measuring the resistivity of a liquid which comprises four electrodes immersed in said liquid, a voltage source connected to a rst pair of said electrodes for driving an alternating current of controlled frequency through said liquid, a resistance in circuit with said first pair of electrodes, means for determining the magnitude of said current and the potential drop between the second pair of said electrodes including an amplier having an output circuit and an input circuit which includes means for deriving an alternating current voltage from said source, a potentiometer, and means for selectivelyr including either said resistance or said second pair of electrodes with the potential thereacross in phase opposition to the potential across said potentiometerwhereby variation of said potentiometer produces a voltage null in said input circuit, a direct current feedback circuit for increasing the overall gain of said amplier as said null point is approached by variation of said potentiometer, and an alternating current negative feedback current having means for sustaining in said feedback circuit all frequencies except those from said source to permit high sensitivity at said null only to the frequency of said source, and an indicator connected in the output circuit for detecting said null.

7. A system for measuring the resistivity of a liquid which comprises a multi-electrode cell containing said liquid immersing the electrodes of said cell, a voltage source of predetermined frequency for driving an alternating current through cuit with said rectier for controlling the gain of I said amplifier as an inverse function of said unbalance signals, an alternating current feedback path having means therein for eliminating signals of the frequency of said source and for sustaining all other signals efiectively to reduce the amplification of said amplier to all signals except from said source whereby the sensitivity of said system is high at said null points exclusively with respect to signals of said frequency.

PAUL P. REICHERTZ.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,004,569 Davis June 11, 1935 2,280,075 Hayward Apr. 21, 1942 2,416,949 Perley et al Mar. 4, 1947 

